Various kinds of speed control systems, commonly called "cruise" control systems, have been employed in the past for automatically maintaining a desired vehicle speed. Such control systems characteristically include some type of vehicle or engine speed sensor that monitors actual vehicle or engine speed which the control system then compares to the vehicle speed at which automatic control is desired which is commonly manually pre-set into the control system to provide an error signal to an actuator connected to the vehicle throttle or other means operative to control engine speed. Accordingly, the error signal progressively diminishes as the actual vehicle or engine speed approaches the desired vehicle or engine speed enabling the actuator to substantially maintain the vehicle or engine speed at the desired condition.
An example of one type of speed control system for trucks that employs a vehicle or engine speed sensor and is operative to compare such speed with a desired vehicle or engine speed to provide an error signal operative to control air pressure delivered to an air actuated throttle is disclosed in U.S. Pat. No. 4,286,685, the disclosure of which is incorporated herein by reference and which further discloses the well known practice by which means are included for automatically disengaging the speed control whenever the brake and/or clutch are employed
In addition to speed control systems, various systems have heretofore been employed to optimize engagement of vehicular transmission gear shift ratios for particular vehicle or engine speeds such as disclosed in U.S. Pat. Nos. 3,776,048 and 4,038,889, the disclosure of both of which are incorporated herein by reference.
Vehicular transmissions are generally provided with a plurality of forward gear ratios predetermined as optimal for use within prescribed speed ranges. The efficient operation of trucks, particularly heavy duty trucks, is highly sensitive to the particular gear ratio being used in relation to the speed and load being carried.
Although the prior art is replete with systems for controlling selection of vehicular transmission gear ratios in relation to engine speed and automatically controlling vehicle or engine speed at a desired condition, no one heretofore had thought to make transmission gear ratios a parameter in determining whether an operator would be able to enjoy the convenience and fuel economy of using automatic speed control.
Since automatic speed control is a convenience to the operator, a need has existed to improve efficient operation of vehicles, particularly trucks and more particularly heavy duty trucks, to provide a means of preventing the operator from engaging the speed or cruise control system in the event the transmission gear ratio is not that predetermined as optimum for the vehicular speed at which automatic control is desired